1. Field of the Invention
The present invention relates to an image recording device, more particularly, to an image recording device applied to a printer, a facsimile device, a copying machine, a display board with an image recording function, or the like and, more particularly, to an image recording device which forms image by developing electrostatic latent image generated using pyroelectric effects with a charged coloring medium.
2. Description of the Related Art
Several image recording methods have been disclosed which are realized by forming electrostatic latent image by using a pyroelectric material which will be heated to generate a charge on its surface and then developing the image with a charged coloring medium.
Bergman et al., for example, propose in U.S. Pat. No. 3,824,098 a copying device which employs a method of selectively heating a pyroelectric material (polyvinylidene fluoride (PVDF)) by lamp light permeated through the original and developing the material with coloring particles (toner). Bergman et al. note latent image generation by charges of reverse polarity in "Applied Physics Letters", Vol. 21(10), 1972, pp. 497-499. More specifically, if electric charges on the surface of a pyroelectric material generated immediately after heating (or during heating) are neutralized, electric charges whose polarity is opposite to that at the heating will be generated on the surface of the pyroelectric material when the pyroelectric material is returned to the room temperature. Latent image formed by thus obtained electric charges of reverse polarity has an advantage that it can be maintained stable in terms of time as compared with that formed by charges generated during heating. Latent image formed by the foregoing process will be hereinafter referred to as "latent image formed by a charge of reverse polarity".
Disclosed in Japanese Patent Laying-Open (Kokai) No. Showa 56-158350 is an image recording device using laser beam or a thermal head as a means of heating a pyroelectric material. In a case where this image recording device uses a thermal head, the thermal head is provided in contact with the surface of the pyroelectric material to conduct selective heating according to an image pattern, thereby generating latent image.
Furthermore, Snelling discloses in U.S. Pat. No. 5,185,619 and Japanese Patent Laying-Open (Kokai) No. Heisei 5-134506 an image recording device which conducts latent image generation, with a heating needle in contact with the surface of a pyroelectric material.
With reference to FIG. 6, basic structure of the image recording device proposed by Snelling will be described in brief. A belt-formed latent image charge holding medium 600 on which latent image is formed is composed of a pyroelectric layer 601 and a conductive layer 602. A heating needle 605 placed in contact with the pyroelectric layer 601 is controlled by a controller 608 to selectively heat the surface of the pyroelectric layer 601 in response to a picture signal. Provided on the surface of the heating needle 605 is a conductive layer 607 grounded as a charge neutralization means through which charges generated on the surface of the pyroelectric layer 601 by heating are neutralized. When the latent image charge holding medium 600 is cooled, charges of reverse polarity are generated to form latent image 640. The formed latent image 640 is developed by a development unit 610 to become toner image 641 and then the toner image is transferred to a recording medium 630 by a transfer means 620 (the Snelling device makes use of the pyroelectric effects also for this transfer means) to form image 642.
As described in the foregoing, in a case where a contact-type heating means such as a thermal head or a heating needle is employed as a heating means, conventional devices conduct heating with the heating means in contact with the surface (surface on which latent image is formed) of a latent image charge holding medium. In such a device, however, repetition of image recording is liable to cause spots on the surface of a heating means or a charge neutralization means, resulting in preventing satisfactory image recording.
In other words, small amounts of toner which has not been transferred to printing paper, paper powder, dust, etc. exist on the surface of the latent image charge holding medium and they will be gradually accumulated on the surface of the heating means or the charge neutralization means to generate spots.
In the device shown in FIG. 6, for example, spots are generated on the surface of the conductive layer 607 or the heating needle 605. Spots generated on the heating means cause problems such as a) heat resistance between the heating means and the latent image charge holding medium is increased to deteriorate heating efficiency and b) conductivity of the surface of the charge neutralization means is reduced to prevent achievement of a sufficient charge neutralization function, resulting in making stable formation of latent image difficult.
Since toner in common use has thermo-fusibility, the toner attached will be fused on the surface of a heating means due to heating of the heating means and its removal is difficult even with a cleaner etc.
Second shortcoming of the conventional devices is that since the heating means and the charge neutralization means are placed in contact with the surface of the latent image charge holding medium, stable formation of latent image is difficult and the latent image charge holding medium has a short life.
More specifically, the conventional system in which the heating means and the charge neutralization means are placed in contact with the surface of the latent image charge holding medium causes such problems as c) sliding caused by contact generates frictional electrification to disorder latent image and d) the surface of the latent image charge holding medium is scratched to disorder latent image or deteriorate durability of the latent image charge holding medium, which are bottlenecks to ensuring of device performance or reliability.
Furthermore, for color recording by the superposition of coloring particles, formation of latent image and a development process should be repeated a plurality of times on the latent image charge holding medium for every kind of coloring particles. However, a plurality of times of latent image formation and a plurality of times of execution of a development process can not be repeated in succession because spots on the surfaces of the heating means and the charge neutralization means or frictional electrification on the surface of the latent image charge holding medium will be generated as mentioned above. It is therefore necessary to once transfer latent image formed halfway by coloring particles to an intermediate transfer medium and conduct latent image formation by the following coloring particles while removing spots on the surfaces of the heating means and the charge neutralization means and frictional electrification on the surface of the latent image charge holding medium. Superposition of coloring particles thus requiring transfer to the intermediate transfer medium makes realization of small-scale and low-cost color recording difficult.